34 



PRINCIPLES OF ANIMAL BIOLOGY 



formed. In chloi'ine, which has seven of the possible eight electrons in its 

 outer shell, two atoms combine by sharing two electrons (one furnished 

 by each atom, Fig. 24), thus making a molecule of chlorine. 



Two atoms of different elements may combine, for the same reason, 

 and thus a compound is produced. Sodium (Na), for example, has just 

 one electron in its outer (third) shell, which it readily gives up to any 

 other atom capable of accepting it. Chlorine, as just explained, has 

 seven in its outer shell and readily accepts an electron from an outside 

 source. The two atoms perform these easy reactions by combining; 

 they form a new substance, sodium chloride (NaCl). 



Valence. — The number of electrons which an atom readily gives up 

 or acquires constitutes its valence. Sodium has a valence of one, since 



Fig. 24. — Two atoms of chlorine combined to complete their outer shells of elections. 



it easily loses but one electron. Magnesium easily loses two electrons, 

 because that is the number in its outer shell, and its yalence is two. 

 These valences must be matched when compounds are formed. Thus, 

 while one atom of chlorine (whose valence is one) matches one of sodium, 

 it requires two atoms of chlorine to take up the two extra electrons of 

 magnesium and form magnesium chloride (MgCU). 



Ions. — When a sodium atom gives up its one outermost electron to 

 some other atom, its electric balance is disturbed. It has lost one unit 

 of negative electric charge; hence the net charge of the remainder is 

 positive. Such a positively charged body is no longer the element 

 sodium, it is not even an atom; it is instead an ion (Na+). Similarly, 

 when a chlorine atom acquires one extra electron (which is, of course, 

 negative), its electric balance is disturbed and it becomes negative. It 

 is no longer chlorine, no longc i- an atom, but a chloride ion (Cl~). An 



